A nuclear reactor such as a boiling water reactor (BWR) includes a reactor core which generates heat for boiling water to generate steam which is used for powering a turbine-generator, for example. The reactor also includes a pressure vessel partially filled with water and containing an annular core shroud therein which surrounds and supports the reactor core. For example, a bottom core plate may be bolted to the core shroud for supporting the core thereto.
The core shroud is spaced radially inwardly from the pressure vessel to define an annular downcomer through which recirculation water is channeled downwardly to the bottom of the pressure vessel where it is turned upwardly for flow through the core. Disposed at the bottom of the downcomer is an annular pump deck which is fixedly joined at its outer perimeter to the pressure vessel and at its inner circumference to the core shroud typically by being welded thereto or by being bolted thereto.
Accordingly, the bottom core plate may be bolted to the core shroud, and the core shroud itself may be bolted to the pump deck by conventional bolts which are relatively large and relatively heavy for handling the large loads channeled therethrough. For example, these bolts may be about 5 cm in diameter and about 45 cm in length, with each bolt weighing about 10 kg.
In the environment of the nuclear reactor, safety considerations require that each of the bolts, including its several components, is fixedly attached to the core shroud or pump deck to prevent the liberation of any part thereof during operation of the reactor, which liberated part could result in damage to other parts of the reactor.
Since these large bolts are used in a reactor, they are usually low-volume parts made to specification, and are not, therefore, mass produced. To prevent excessive cost of manufacture, the bolts are typically made from threaded studs, with corresponding nuts being disposed at both ends of the stud for clamping together the required components. One of the nuts is, therefore, typically welded to the stud to prevent its disassembly therefrom, and it is also welded to one of the members or plates being bolted together to prevent its liberation therefrom. The other nut is suitably tightened on the stud for clamping together the respective members or plates, and is prevented from loosening by a conventional hexagonal socket keeper which surrounds the nut and which is itself welded to the other member being clamped together. In the event of failure of the stud by a fracture therethrough at any axial location between the two nuts, each of the nuts is fixedly joined to its respective plate and each of the severed stud halves is retained by the respective nut. In this way, neither the nuts themselves nor the stud segments are liberated from the joint and therefore cannot be entrained with the recirculation flow to cause damage to parts of the reactor downstream therefrom.
However, a fastener arrangement such as that described above including a stud and two corresponding nuts at opposite ends thereof requires suitable access to both sides of the stud on both sides of the members being clamped together. In a nuclear reactor, space is typically limited within the pressure vessel making it difficult if not impossible to access both nuts for assembly and welding especially after buildup of the reactor. It is desirable to provide an improved fastener assembly which is captive in the reactor but which also allows improved assembly and disassembly thereof both for initial buildup of the reactor as well as for subsequent maintenance outages in which components such as the core shroud or bottom core plate are removed for replacement. Or, for replacement of the fasteners themselves either for preventive maintenance or upon failure thereof.